Endovascular stents are coated frequently with a polymer that contains one or more therapeutic substances within a polymeric matrix to improve the efficacy of the stents. These substances are eluted from the stent coating to the tissue bed surrounding the implanted stent. The effectiveness of these therapeutic substances is generally improved because localized levels of medication may be higher and potentially more successful than orally or intravenously delivered drugs, which are distributed throughout the body rather than concentrated at the location of most need. Drugs released from tailored stent coatings may have controlled, time-release qualities, eluting their bioactive agents over hours, weeks or even months.
Various methods of coating a stent or other implantable medical device with one or more polymers containing one or more therapeutic substances are known. For example, a common solvent or a pair of solvents may be used to dissolve drugs and polymers, including copolymers, terpolymers or polymer blends. The resulting drug-polymer solution is then applied on the stent by spraying, dipping, brushing, or rolling. The stent is then dried, for instance in a vacuum or oven, to evaporate the solvent, leaving the therapeutic substance or therapeutic substance and polymer coating on the stent. Problems may arise in getting polymer coatings to adhere to stents, particularly stents made of cobalt-based alloys. Most coronary stents are finished by electrochemical polishing for surface smoothness. A smooth surface is desirable because early research has shown that a stent with a rough surface results in more platelet cell adhesion, thrombus, inflammation, and restenosis than a smoothly polished stent. The smooth surface may pose a challenge to the coating, however. Due to the very different nature of the polymer and the metallic substrate, a polymeric coating does not easily adhere to the metallic substrate. If the coating does not adhere well to the metal surface, it may cause problems such as coating delamination, irregular drug release profiles, or embolism caused by broken and detached debris from the coating.
It is also known in the art to coat a stent with a polymer that does not contain a therapeutic substance, for example, to form a sealant overcoat layer or primer coating. Methods of applying a polymer coating include vapor phase deposition. Vapor phase deposition typically entails vaporizing a dimer, and applying energy to the vaporized dimer to create an active monomer. The system set up is such that the monomer is deposited onto the stent and polymerizes in-situ, thereby creating a polymer coating around the stent. However, vapor deposition is not used for applying a therapeutic substance coating because the high temperatures associated with the process may cause the drugs to break down and lose their efficacy.
The present invention is related to an improved drug coating for an implantable medical device.